The invention is based on a method for coating at least one wiper blade element.
Wiper blade elements for vehicle window wipers are produced in mass quantities, typically more than ten thousand per day. They are cut from a rubber strip that has been extruded and then vulcanized.
DE 198 14 805 A1 has disclosed a method for coating a wiper blade element, in which at least one protective coating is produced on the surface of the extruded, elastomer material of the wiper blade element by means of a CVD process (Chemical Vapor Deposition) in order to improve the abrasion and sliding properties of the wiper blade element. So that the protective coating adheres better to the base material, the surface of the wiper blade element is pretreated by being cleaned and activated with a plasma. A layer of bonding agent can also be applied.
It is also known that a bias voltage applied to the wiper blade element causes the plasma-aided depositing of the protective coating to produce denser and more abrasion resistant coatings. In this connection, the wiper blade element is conveyed past an electrode to which a bias voltage is applied. This bias voltage, also referred to as priming voltage, is supplied pulsed or unpulsed in opposition to ground or a counter-electrode. In this connection, pulse frequencies of between 10 kHz and a few MHz are used, preferably from 50 to 250 kHz. The bias voltage can, however, also be supplied by a frequency source at frequencies between 1 kHz and 100 MHz, preferably from 50 kHz to 27 MHz, in particular 13.56 MHz. The bias voltage accelerates ions from the plasma in the direction of the surface of the wiper blade element. These ions strike the previously deposited coating and cause a cross-linking and densification of the coating. The bias voltage advantageously occurs automatically and can assume a value between a few volts and 2 kV.
In order to be able to treat and coat mass quantities of wiper blade elements, the coating is preferably integrated into a continuous production flow, which can save space, time, and expense. Continuous concepts particularly suited for this are those in which the wiper blade elements are extruded in a long strand and are conveyed through the differentially pumped vacuum chamber or reaction chamber at atmospheric pressure, past the coating sources. The coupling of two critical manufacturing steps, namely of extrusion and coating of the wiper blade element, however, causes the entire production line to come to a halt when there is a breakdown in one part of the manufacturing system and generates a greater amount of manufacturing rejects. In addition, the protective coating at the ends of the wiper blade element can be damaged in the final cutting procedure.
EP 0 264 227 A2 has also disclosed a wiper blade element for window wipers which is comprised of a high-molecular material. The group of materials includes, among others, natural rubber, synthetic rubber, elastomers, synthetic plastics, and the like. The surface of the wiper blade element is given a polymer protective coating, which is formed directly from a monomeric gas through the use of plasma energy.
According to the invention, the wiper blade element is cut to a usable length from a profiled band before being brought into a treatment chamber, is placed on the electrode plate, and is subjected to a plasma flow.
The individual treatment steps are advantageously executed one after another in a treatment chamber in a so-called single-chamber batch process or in mass quantities in a multiple-chamber batch process with a number of treatment chambers. In a functional manner, a single treatment step is executed in each chamber, where cycle times between the individual treatment steps can be between 10 seconds and several minutes. Cycle times between 30 seconds and 1 minute have turned out to be particularly favorable. The batch process permits the production-critical processes of the rubber shaping and the coating to be decoupled. If malfunctions occur in one manufacturing process, this has no influence on the other manufacturing process. As a result, the risk of rejects is also reduced. In contrast to continuous systems, a coating system with a number of treatment chambers can be embodied in a more favorably priced, flexible manner since a number of similar treatment chambers can be operated in parallel, depending on the treatment duration. This produces particularly positive results if the pretreatment takes place at a slight vacuum of approximately 0.1 to 100 mbar.
The wiper blade elements can be suitably cut from an extruded double-strand profile in which the wiper lips are oriented toward each other and connected to each other via an intermediary piece. One side of the wiper lips rests against the electrode plate. After the first side is coated, the wiper blade element must be turned over inside or outside of the treatment chamber. When the coating is completed, the wiper lips are cut from the intermediary piece. It is also possible to cut the wiper lips apart from each other before the coating.
In a variant of the invention, the wiper blade elements are arranged so that the wiper lips stand approximately perpendicular to the electrode plate, which extends on both sides of the wiper blade element. Thus, both sides of the wiper lip of the wiper blade element can be treated and coated at the same time. A number of wiper blade elements can be arranged next to one another and in series on an electroplate, which represents a part of a merchandise carrier. They can be of different lengths and have different profiles.
The electrode plate suitably engages laterally in longitudinal grooves of the wiper blade element, which are provided for claw brackets or spring strips, so that the wiper blade elements are securely fixed to the electrode plate. This makes a flexible large-scale manufacture possible. Furthermore, cutting processes after coating, which could damage the protective coating, can be eliminated.
The electrode plate suitably has an alternating current applied to it, with a frequency of 10 kHz to a few MHz, preferably 13.56 MHz, where the power to be coupled-in is approximately 1 to 100 W/cm2 of electrode surface area. Depending on the coupled-in power, the overall pressure, and the surface area ratios, a negative potential (self bias) occurs in the plasma between the electrode plate and ground. This potential engages through the rubber of the wiper blade element and accelerates the ions in the plasma toward the surface of the rubber. This ion bombardment causes a compaction of the coating and thus increases the coating hardness. The effectiveness of the ion bombardment depends on the thickness of the dielectric disposed on the electrode plate, i.e. the thickness of the elastomer material.
An increase in the distance between the electrode and the rubber surface decreases the potential induced on the surface by the self bias voltage. Through the invention""s disposition of the wiper blade element on the electrode plate, the ion bombardment and the resulting coating hardness vary as a result of the varying distance of the wiper lip and the fastening part from the electrode plate. Thus, a hardness and coating thickness gradient can be produced, which ranges from a softer coating on the wiper lip to a harder coating on the profiled back. However, the thickness and hardness of the protective coating directly on the wiper lip are crucial to the functionality of the wiper blade element. The coating thickness and coating hardness that occurs on the fastening part have only a negligible impact on the wiping process. Since the potential occurring in the electrode plate reaches both sides of the wiper lip, both sides of the wiper lip can advantageously be coated at the same time. In order to influence the negative potential that occurs and to optimize it with regard to the respective sort and type of wiper blade element, one embodiment of the invention proposes fully or partially covering the electrode plate with an insulating material. Thus, the area in contact with the plasma can be modified and the self bias voltage occurring in the electrode plate can be adjusted.
The gas is introduced into the treatment chambers by means of gas nozzles; the openings of the nozzles are oriented toward the wiper lips of the wiper blade elements introduced. Low-molecular, cross-linkable, gaseous materials, halogen-containing, silicon-containing, carbon-containing, or metal-organic monomers are suitable as coating materials. One or more gas nozzles can be associated with one or more wiper blade elements in order to assure a uniform coating of a batch. In addition, conveying apparatuses for the gas can be provided in the treatment chambers. These can be comprised in that gas slots are disposed at the longitudinal sides of the wiper blade elements, lateral to the electrode plates, or in the electrode plates, through which the gas is aspirated by a vacuum pump. In addition, gas baffles can also be disposed in the treatment chamber.
If a number of treatment chambers are provided, they can be arranged in a line and interlinked with one another by means of a transport mechanism. Depending upon the availability of space, it can be advantageous to arrange a number of treatment chambers in a closed configuration. In this connection, it is possible to use one treatment chamber for loading and unloading the merchandise carriers with the wiper blade elements.